To answer the question What is an exoplanet? it is useful to first consider the planets in our Solar System, and to determine what sets these well-known bodies aside from other entities such as the Sun and Pluto. You may consider this as being flung in the deep end, but it is an important first step to appreciate what an exoplanet- short for extra-solar planet- really is!
As of 2006, the International Astronomical Union (IAU) define a celestial body as a planet if:
- It is in orbit around the Sun;
- It has enough mass for its self-gravity to overcome rigid body forces, so that it compacts down into the smallest space possible- a sphere (or more accurately into a roughly spherical shape). This defines a rough lower mass limit for a planet;
- It has cleared its orbital neighbourhood. In other words, there are no other bodies of a similar or significant size nearby.
It is this last point which led to the eventual declassification of Pluto as a planet. Pluto certainly meets the first two points outlined by the IAU, but there are too many objects near to Pluto in our Solar System which are of similar size and mass (including the dwarf planet Eris which is 27% heavier than Pluto).
This brings us to the definition of an exoplanet: simply put, an exoplanet is a planet orbiting any star other than our Sun. Although there is no recognised definition for an exoplanet in the same way as above, this stands as a good guide.
What is the difference between a planet and a star? This is a question we have heard many times over the years at Kielder Observatory, and it is a lot more intuitive than you might think! Stars like our Sun- and indeed this is the case for all of the stars that hang in the night sky- are big burning balls of gas, and generally more massive (heavier). They are powered by a process called nuclear fusion, which in its most basic form is where two hydrogen atoms are squeezed together under immense temperatures and pressures to produce helium. This gives off enormous amounts of energy which is the fuel for all stars. However, this process is not occurring inside planets because they are not massive enough for their cores to reach such extreme conditions. In fact, if you were to add around 100 Jupiters to Jupiter, the result would be a small star (you could say Jupiter is a failed star).
There exists, then, an upper and lower mass limit for what constitutes an exoplanet, although this is a topic of debate in scientific circles. In summary, an exoplanet is simply a planet orbiting a star outside of our Solar System, and to date there are over 4000 which have been found.